Circuit used for power factor correction

ABSTRACT

A circuit ( 1 ) used for power factor correction, corresponds to a known circuit that includes a high-frequency frequency source ( 15 ) for generating a high-frequency voltage (Uf) for a pump capacitor ( 14 ). The high-frequency voltage source ( 15 ) has two on/off switches ( 151, 152 ) connected in series that generate, at an electric connection point ( 153 ), a square wave voltage (Ul) that varies between these two switches with a frequency (f) of at least 1 megahertz. The voltage source further includes a smoothing device ( 150 ) for generating from the square wave voltage a smoothed voltage (Uf) for the pump capacitor, whereby the smoothed voltage varies with a frequency (f) of at least 1 megahertz.

The invention relates to a circuit for power factor correction accordingto the preamble of claim 1 and as is known from Jinrong Quian et al.:“Charge Pump Power-Factor-Correction Technologies Part I: Concept andPrinciple”, IEEE Transactions on Power Electronics, Vol. 15, No. 1,January 2000 (with particular reference to FIG. 6 a therein andaccompanying description on page 123, right-hand column and subsequentpages).

This known circuit and other known circuits for power factor correction(see e.g. the cited document, U.S. Pat. No. 4,808,887, No. 5,008,597,No. 5,371,440, No. 5,521,467, No. 5,789,871, No. 5,914,572, No.6,057,652, No. 6,091,206, DE-A-3 142 613, WO 97/22231, WO 99/25159,JP-A-10-214 695, JP-A-2000-012 257) are currently used, for example, inmodern electronic ballasts, which convert the electrical power from theavailable mains voltages, usually alternating voltages, in such a waythat each of the connected loads, mainly lamps, can be operated in theiroptimum voltage, current and frequency range. With respect to the mainssystem, the electronic ballasts are meant to exhibit an electricalresponse corresponding as closely as possible to a resistance in orderto avoid distortions in the mains current and the mains voltage.

The known circuits for power factor correction work at frequencies of 20to 100 kHz. The frequency is the determining factor for the size of theinductors and capacitors that are required, which are by far the largestcomponents of such circuits.

The invention is based on the problem of providing a circuit for powerfactor correction that enables miniaturization of this circuit.

This problem is solved by a circuit for power factor correction havingthe features specified in claim 1.

According to this solution, a circuit for power factor correction isprovided that comprises:

-   -   an input terminal for applying an electrical power having a        direct voltage that varies in particular over time with respect        to an electrical reference potential,    -   a reference-potential terminal for applying the reference        potential    -   an output terminal for drawing a power-factor corrected        electrical power,    -   at least two diodes connected in series between the input        terminal and output terminal, each connected in the forward        direction from the input terminal to the output terminal,    -   a capacitor connected between the output terminal and the        reference-potential terminal,    -   an extra capacitor having an electrode connected to an        electrical junction of the two diodes, and another electrode,        and    -   a frequency voltage source for generating an output voltage        which is applied to the other electrode of the extra capacitor,        said output voltage varying substantially between the direct        voltage at the input terminal and the reference potential at the        reference-potential terminal at a frequency that is higher than        a frequency of the direct voltage at the input terminal, and        which is characterized in that the frequency voltage source        comprises:    -   two on/off switches connected in series between the output        terminal and the reference-potential terminal, which switch in        push-pull mode at such high speed at a frequency of at least 1        Megahertz that at an electrical junction of these two switches a        square-wave voltage is generated that varies substantially        between the direct voltage at the input terminal and the        reference potential at the reference-potential terminal at this        frequency of at least 1 Megahertz, and    -   a smoothing device for smoothing the square-wave voltage and        generating from the square-wave voltage a smoothed voltage which        is applied as output voltage of the frequency voltage source to        the other electrode of the extra capacitor, said smoothed        voltage varying substantially between the direct voltage at the        input terminal and the reference potential at the        reference-potential terminal at the frequency of at least 1        Megahertz, where    -   a capacitance of the extra capacitor is selected so that the        extra capacitor is charged and/or discharged at the frequency of        at least 1 Megahertz of the smoothed voltage.

In the circuit according to the invention, the miniaturization isadvantageously achieved by the significant increase in switchingfrequency from the kHz range to the MHz range. At the same time, thecircuit according to the invention advantageously enables switchingoperation in the MHz range at a high efficiency of 85% to 95%, forexample, and with good power factor correction using the on/off switchesswitching at ultrahigh speed.

In the known circuits for power factor correction operated at 20 to 100kHz, these moderately high frequencies do mean that a variety ofcircuits with high efficiency and good power factor correction can berealized, but the size can only be reduced to a limited extent owing tothe inductors and capacitors that are required (for a given power).

In a preferred and advantageous embodiment of the circuit according tothe invention, the smoothing device comprises a low-pass filter that issubstantially completely transparent for the frequency of at least 1Megahertz, to which the square-wave voltage from the junction of theswitches is fed as an input signal, and whose output signal constitutesthe smoothed voltage that is applied as output voltage of the frequencyvoltage source to the other electrode of the extra capacitor.

In a preferred and advantageous form of this embodiment of the circuitaccording to the invention, the low-pass filter comprises two inductorsconnected in series between the other electrode of the extra capacitorand the junction of the two switches, and an additional capacitor thatis connected between the reference-potential terminal and an electricaljunction of the inductors.

In an additional preferred and advantageous embodiment of the circuitaccording to the invention, the smoothing device comprises an RFtransformer having a primary inductor connected between thereference-potential terminal and the junction of the two switches, andhaving a secondary inductor connected between the other electrode of theextra capacitor and the reference-potential terminal and coupled to theprimary inductor. In this embodiment, the RF transformer is preferablyoperated at resonance with a coupling coefficient k<1.

In a preferred and advantageous form of this further embodiment, anadditional capacitor is connected between the primary inductor and thejunction of the two switches.

The one and additional embodiment of the circuit according to theinvention can be combined with one another in a single circuit, inparticular also in the specified forms.

Advantageously, the efficiency of the circuit according to the inventioncan be further optimized if an additional extra capacitor is connectedin parallel with the extra capacitor between the reference-potentialterminal and the junction of the two diodes. This additional extracapacitor can be used as a tuning capacitor for optimizing theefficiency.

An on/off switch of the circuit according to the invention preferablycomprises a MOS transistor switch, in particular a CoolMOS® switch.

The circuit according to the invention advantageously contains only afew components in all embodiments and their forms, while advantageouslyachieving a high efficiency of far greater than 90% in the MHz frequencyrange with very good power factor correction (PFC), providedRF-compatible capacitors, inductors and diodes (Schottky diodes) areused. Its design can be advantageously miniaturized because RF inductorsand RF capacitors can be kept very small.

The circuit according to the invention is particularly suitable forelectronic ballasts, in particular for any type of lamp.

The invention is described in more detail below with reference to thefigures, in which:

FIG. 1 shows a first exemplary embodiment of the circuit according tothe invention, and

FIG. 2 shows a second exemplary embodiment of the circuit according tothe invention.

In the figures, the circuit according to the invention for power factorcorrection is referred to in general by 1.

An input terminal 10 of the circuit 1 is used for applying an electricalpower P having a direct voltage U that varies in particular over timewith respect to an electrical reference potential φ that is to beapplied to a reference-potential terminal 11 of the circuit 1, and whichis ground for example.

For example, the direct voltage U is a pulsed direct voltage of e.g. 220V and frequency f₀=50 Hz, generated from a mains alternating voltage byrectification in a rectifier that is not shown.

The electrical power P1 that has undergone power factor correction inthe circuit 1 is to be drawn from an output terminal 12, for example bya load 2 consisting of one or more lamps for instance, connected betweenthe output terminal 12 and the reference-potential terminal 11. In thefigures, this load 2 is connected, for example, between the outputterminal 12 and an additional output terminal 13 of the circuit 1 thatis connected directly to the reference-potential terminal 11, and likethis terminal is at the reference potential φ.

Two diodes 21 and 22 are connected in series between the input terminal10 and the output terminal 12, each connected in the forward directionfrom the input terminal 10 to the output terminal 12, so that electricalcurrent I can flow from the input terminal 10 through the diodes 21 and22 to the output terminal 12 when there is a direct voltage U at theinput terminal 10.

A capacitor 13, also called a charging capacitor, is connected betweenthe output terminal 12 and the reference-potential terminal 11, and hasa sufficiently high capacitance C_(lade).

An extra capacitor 14 has an electrode 141 connected to an electricaljunction 20 of the two diodes 21 and 22, and another electrode 142. Thisextra capacitor 14, also called a pump capacitor, has a capacitanceC_(pump) that is smaller than C_(lade).

A frequency voltage source 15 is used for generating an output voltagewhich is applied to the other electrode 142 of the extra capacitor 14,said output voltage varying substantially between the direct voltage Uat the input terminal 10 and the reference potential φ at thereference-potential terminal 11 at a frequency that is higher than afrequency, e.g. the frequency f₀, of the direct voltage U at the inputterminal 10.

Up to this point, the circuit 1 according to the invention correspondsto the circuit that follows from the first document cited above.

In the circuit 1 according to the invention, the frequency voltagesource 15 comprises two on/off switches 151 and 152 connected in seriesbetween the output terminal 12 and the reference-potential terminal 11,which switch in push-pull mode at such high speed at a frequency f of atleast 1 Megahertz that between these two switches 151 and 152 asquare-wave voltage Ul is generated that varies substantially betweenthe direct voltage U at the input terminal 10 and the referencepotential φ at the reference-potential terminal 11 at this frequency fof at least 1 Megahertz.

If the lower switch 152 is on while the upper switch 151 is off, at thismoment in time the junction 153 is at the voltage equal to the voltagepresent at the output of the diode 22, and hence present at the outputterminal 12 of the circuit 1, said voltage being substantially equal tothe direct voltage U at the input terminal 10 of the circuit 1. If onthe other hand the lower switch 152 is off while the upper switch 151 ison, at this moment in time the junction 153 is at a voltagesubstantially equal to the reference potential φ.

In addition, in the circuit 1 according to the invention, the frequencyvoltage source 15 comprises a smoothing device 150 for smoothing thesquare-wave voltage Ul and for generating from the square-wave voltageUl a smoothed voltage Uf which is applied as output voltage of thefrequency voltage source 15 to the other electrode 142 of the extracapacitor 14, said smoothed voltage varying substantially between thedirect voltage U at the input terminal 10 and the reference potential φat the reference-potential terminal 11 at the frequency f of at least 1Megahertz.

The smoothing device 150 is preferably designed so that a substantiallysinusoidal smoothed voltage Uf of the frequency f is generated from thesquare-wave voltage Ul of the frequency.

The capacitance C_(pump) of the extra capacitor 14 is selected in thecircuit 1 so that the extra capacitor 14 is charged and/or discharged atthe frequency f of at least 1 Megahertz of the smoothed voltage Uf.

In the exemplary embodiment shown in FIG. 1 of the circuit 1, thesmoothing device 150 comprises a low-pass filter 160 that issubstantially completely transparent for the frequency f of at least 1Megahertz. The square-wave voltage Ul from the junction 153 of theswitches 151 and 152 is fed as an input signal to the low-pass filter160, and the output signal of the low-pass filter 160 constitutes thesmoothed voltage Uf that is applied as the output voltage of thefrequency voltage source 15 to the other electrode 142 of the extracapacitor 14.

The low-pass filter 160 especially comprises two inductors 161 and 162connected in series between the other electrode 142 of the extracapacitor 14 and the junction 153 of the two switches 151 and 152, andan additional capacitor 163. This capacitor 163 is connected between thereference-potential terminal 11 and an electrical junction 164 of theinductors 161 and 162.

In the exemplary embodiment shown in FIG. 2, the smoothing device 150comprises an RF transformer 170 having a primary inductor 171 connectedbetween the reference-potential terminal 11 and the junction 153 of thetwo switches 151 and 152, and having a secondary inductor 172 connectedbetween the other electrode 142 of the extra capacitor 14 and thereference-potential terminal 11 that is coupled to the primary inductor171, for example via an iron core 174.

An additional capacitor 173 is especially connected between the primaryinductor 171 and the junction 153 of the two switches 151 and 152, saidadditional capacitor providing DC isolation between the junction 153 andthe transformer 170 and providing only AC coupling to the transformer170. The transformer 170 is operated at resonance with a couplingcoefficient k<1.

In the circuit 1 shown in FIGS. 1 and 2, an additional extra capacitorcan be connected in parallel with the extra capacitor 14 between thereference-potential terminal 11 and the junction 20 of the two diodes 21and 22, as is shown in the exemplary embodiment of FIG. 2 with thereference 14′.

The efficiency of the circuit 1 can be further optimized using such anadditional extra capacitor 14′.

Each of the on/off switches 151 and 152 is implemented by a MOStransistor switch, preferably a CoolMOS® switch.

To summarize, the circuit 1 for power factor correction corresponds to aknown circuit of this type having an RF voltage source 15 for generatinga radio-frequency voltage Uf for a pump capacitor 14. The RF voltagesource 15 comprises according to the invention two series-connectedon/off switches 151 and 152, which generate at an electrical junction153 of these two switches 151 and 152 a square-wave voltage Ul thatvaries at a frequency f of at least 1 Megahertz, and a smoothing device150 for generating from the square-wave voltage Ul a smoothed voltage Uffor the pump capacitor 14, said smoothed voltage varying at thefrequency f of at least 1 Megahertz.

1. A circuit (1) for power factor correction comprising: an inputterminal (10) for applying an electrical power (P) having a directvoltage (U) that varies in particular over time with respect to anelectrical reference potential (φ), a reference-potential terminal (11)for applying the reference potential (φ) an output terminal (12) fordrawing a power-factor corrected electrical power (P1), at least twodiodes (21, 22) connected in series between the input terminal (10) andoutput terminal (12), each connected in the forward direction from theinput terminal (10) to the output terminal (12), a capacitor (13)connected between the output terminal (12) and the reference-potentialterminal (11), and an extra capacitor (14) having an electrode (141)connected to an electrical junction (20) of the two diodes (21, 22), andanother electrode (142), and a frequency voltage source (15) forgenerating an output voltage which is applied to the other electrode(142) of the extra capacitor (14), said output voltage varyingsubstantially between the direct voltage (U) at the input terminal (10)and the reference potential (φ) at the reference-potential terminal (11)at a frequency that is higher than a frequency (f₀) of the directvoltage (U) at the input terminal (10), characterized in that thefrequency voltage source (15) comprises: two on/off switches (151, 152)connected in series between the output terminal (12) and thereference-potential terminal (11), which switch in push-pull mode atsuch high speed at a frequency (f) of at least 1 Megahertz that at anelectrical junction of these two switches (151, 152) a square-wavevoltage (Ul) is generated that varies substantially between the directvoltage (U) at the input terminal (10) and the reference potential (φ)at the reference-potential terminal (11) at this frequency (f) of atleast 1 Megahertz, and a smoothing device (150) for smoothing thesquare-wave voltage (Ul) and generating from the square-wave voltage(Ul) a smoothed voltage (Uf) which is applied as output voltage of thefrequency voltage source (15) to the other electrode (142) of the extracapacitor (14), said smoothed voltage varying substantially between thedirect voltage (U) at the input terminal (10) and the referencepotential (φ) at the reference-potential terminal (11) at the frequency(f) of at least 1 Megahertz, where a capacitance (c_(pump)) of the extracapacitor (14) is selected so that the extra capacitor (14) is chargedand/or discharged at the frequency (f) of at least 1 Megahertz of thesmoothed voltage (Uf).
 2. The circuit as claimed in claim 1, wherein thesmoothing device (150) comprises a low-pass filter (160) that issubstantially completely transparent for the frequency (f) of at least 1Megahertz, to which the square-wave voltage (Ul) from the junction (153)of the switches (151, 152) is fed as an input signal, and whose outputsignal constitutes the smoothed voltage (Uf) that is applied as outputvoltage of the frequency voltage source (15) to the other electrode(142) of the extra capacitor (14).
 3. The circuit as claimed in claim 2,wherein the low-pass filter (160) comprises two inductors (161, 162)connected in series between the other electrode (142) of the extracapacitor (14) and the junction (153) of the two switches (151, 152),and an additional capacitor (163) that is connected between thereference-potential terminal (11) and an electrical junction (164) ofthe inductors (161, 162).
 4. The circuit as claimed in claim 2, whereinthe smoothing device (150) comprises an RF transformer (170) having aprimary inductor (171) connected between the reference-potentialterminal (11) and the junction (153) of the two switches (151, 152), andhaving a secondary inductor (172) connected between the other electrode(142) of the extra capacitor (14) and the reference-potential terminal(11) and coupled to the primary inductor (171).
 5. The circuit asclaimed in claim 4, wherein an additional capacitor (173) is connectedbetween the primary inductor (171) and the junction (153) of the twoswitches (151, 152).
 6. The circuit as claimed in claim 1, wherein anadditional extra capacitor (14′) is connected in parallel with the extracapacitor (14) between the reference-potential terminal (11) and thejunction (20) of the two diodes (21, 22).
 7. The circuit as claimed inclaim 1, wherein an on/off switch (151, 152) comprises a MOS transistorswitch.
 8. The circuit as claimed in claim 2 or 3, wherein the smoothingdevice (150) comprises an RF transformer (170) having a primary inductor(171) connected between the reference-potential terminal (11) and thejunction (153) of the two switches (151, 152), and having a secondaryinductor (172) connected between the other electrode (142) of the extracapacitor (14) and the reference-potential terminal (11) and coupled tothe primary inductor (171).
 9. The circuit as claimed in claim 4,wherein an additional capacitor (173) is connected between the primaryinductor (171) and the junction (153) of the two switches (151, 152).